Recent years, in various electronic devices, their operating frequencies have increased and their packages (especially, their height) have been downsized. According to such an increase in operating frequency and a reduction in package size, there is also a need for piezoelectric resonator devices (crystal oscillators, for example) to be adaptable to the increase in operating frequency and the reduction in package size.
In this kind of piezoelectric resonator devices, a housing is constituted by a substantially rectangular-shaped package. The package is constituted by: a first sealing member and a second sealing member both made of glass or crystal; and a crystal resonator plate made of crystal. On respective main surfaces of the crystal resonator plate, excitation electrodes are formed. The first sealing member and the second sealing member are laminated and bonded via the crystal resonator plate. Thus, the excitation electrodes of the crystal resonator plate that is disposed in the package (in the internal space) are hermetically sealed (for example, see Patent Document 1). Hereinafter, such a laminated structure of the piezoelectric resonator device is referred to as a sandwich structure.